It is known in the art to apply electric boosting of vehicle turbochargers using an electric motor to quickly accelerate the turbocharger wheels and reduce boost lag during vehicle acceleration. It is desired to apply this and other concepts to positive displacement machines having coacting rotors, such as Roots rotors or screw rotors, for supercharging internal combustion engines. Such a device used as an automotive supercharger may include a housing having a rotor cavity in which a pair of parallel rotors having interleaved lobes rotate to pump or compress air drawn into one end of the housing and discharged through an opening in the cavity wall near an opposite end of the housing. The rotors may be belt driven by the engine through a pulley connected through a gear train to the pair of rotors.